This invention relates to an improvement to the direct coking process for obtaining hydrocarbon products by thermal upgrading of naturally-occurring hydrocarbons chemically bound or physically mixed with mineral solids, such as bituminous sands.
Although the development work to supply details for the embodiment described herein was performed using Athabasca bituminous sands as the feed material, the invention may also find application in the thermal treatment of oil shales. For simplicity and clarity, the invention is described for bituminous sands in particular, although those skilled in the oil shale art will recognize its applicability to this latter material.
Although the bitumen in naturally-occurring bituminous sands is a mixture of hydrocarbons mostly of high molecular weight, and as such is not of great economic value, such bitumen may be upgraded to hydrocarbons of lower molecular weight, mainly liquid at room temperature. These derived hydrocarbons find numerous applications in domestic and industrial use, and in transport. Hence, by suitable thermal treatment, the naturally-occurring bitumen becomes a valuable source of hydrocarbons. In view of the vastness of the deposits of bituminous or tar sands and oil shale, these raw materials are capable of filling energy and chemical feedstock needs resulting from the depletion of conventional crude oil.
A recovery method that has been successfully applied to obtaining useful hydrocarbons from tar sand uses an extraction process commonly known as the hot water process. This process takes advantage of the fact that tar sands, on being mulled with hot water and sodium hydroxide, produces a bituminous slurry, which slurry, on being diluted with further hot water and led to a settling zone, divides such that a bituminous froth rises to the surface of the water in the settling zone and may be withdrawn for further concentration of the bitumen, while sand, essentially bitumen-free, may be discarded as a downward flowing aqueous tailings stream. The bitumen obtained in the froth may be concentrated by diluting the bitumen with a naphtha solvent, after which the diluted froth is centrifuged to remove substantially all water and mineral solids. The naphtha solvent is then removed by distillation, to leave a bitumen essentially free from water, minerals, and solvent.
Once isolated, the bitumen has heretofore been processed by delayed or fluid coking. Such treatment serves three principal purposes. First, it breaks down the larger-sized molecules to species of lower molecular weight and hence higher volatility. It is these smaller molecules that find application as useful and therefore economically-valuable hydrocarbons. This aspect of thermal treatment is commonly referred to as "cracking." Secondly, the thermal treatment removes carbon from the mixture of molecules from which bitumen is composed, thus bringing the ratio of carbon to hydrogen in the mixture to such a level that the mixture consists of hydrocarbons amenable to refining in an established oil refinery. Thirdly, the thermal processing encourages the evolution of nitrogen and sulphur and other elements undesirable in the end use to which the hydrocarbons are to be put.
Although it has gained acceptance at the commercial level, the hot water process has several drawbacks. Principal among these is that the process generates large quantities of wet tailings. As a consequence, a costly tailings pond must be maintained. Another disadvantage arising from high water usage is that large quantities of heat must be used to raise the temperature of the water to the process temperature of 179.degree. F or thereabouts, and thus heat is lost as the water leaves the process in the tailings streams. Recovery of heat is uneconomical since the difference in temperature between the tailings and the surrounds is relatively small.
The idea early suggested itself in tar sand processing that instead of extracting the bitumen and then subjecting it to thermal treatment, it would be feasible and desirable to subject the tar sand feed in its entirety to thermal treatment. This could be done by retorting the bitumen in its natural state, intimately mixed in the tar sand. Early work on such retorting of tar sand is reported by P. E. Gishler and W. S. Peterson in "Oil from Alberta Bituminous Sand" in Petroleum Engineer Vol. 23, Issue 23, pp. c 66-c 76(1951), and "The Fluidized Solids Technique Applied to Alberta Oil Sands Problem" in Proceedings of the Athabasca Oil Sands Conference, Edmonton, Alberta, pp. 207 to 236 (1951), and by R. W. Ramuler in "The Production of Synthetic Crude Oil from Oil Sand by Application of the Lurgi-Rhurgas Process" in the Canadian Journal of Chemical Engineering Vol. 48, pp. 552-560 (1970). The technology of the retorting of tar sand is known as, and is hereinafter referred to as, direct coking.
An important difference between the coking of bitumen and the coking of total tar sand feed is that in the case of tar sand, not only the bitumen, but also the water and the mineral solids in the tar sand must be raised to the temperature of the retorting zone. Hence provision must be made in a direct coking unit to heat the entire in-coming feed to such a temperature that moisture is vapourized, the bitumen is "cracked," and hydrocarbon products are vapourized and driven out of the reaction mixture.
The tailings from direct coking consist essentially of grains of mineral solid surrounded by coke deposit -- all at high temperature. If the coked solids are merely discarded, the process would be so wasteful of heat as to be uneconomical when compared with other systems. To overcome this problem, the coked solids are conveyed to a burning chamber or zone where the coke is burned in the presence of air or oxygen. This has the effect of further raising the temperature of the mineral residue. A portion of the hot minerals may then be returned to the coking reaction zone as a convenient means for supplying the heat needed to raise the temperature of the in-coming tar sand feed. The portion of the burned mineral solids that is not returned to the reaction zone may be conveyed to heat exchangers wherein the heat is extracted, for instance, in the production of steam.
To summarize, the direct coking process involves two stages: a reacting stage, in which the tar sand is heated in the absence of air to give "cracked" hydrocarbon product, with nitrogen and sulphur released by said "cracking" being evolved in the form of nitrogen and sulphur-containing gases such as ammonia, hydrogen sulphide and sulphur dioxide; and a burning stage in which coke-enveloped mineral solids emerging from the reacting stage are burned by the addition of air or oxygen and part of the hot mineral solids are recycled to the reacting stage.
It is the burning step that is the subject of the invention described herein. To keep the process in heat balance, it is necessary to add supplementary fuel in the burning step. That is to say, the heat obtained from burning the coke adhering to the minerals is inadequate to replace the heat lost in cracking the bitumen and distilling out volatiles. For simplicity and efficiency this supplementary fuel is best added in the burning stage of the process so that the heat is in effect added to the mineral solids. This burning of supplementary fuel and burning of coke adhering to mineral particles produces waste gases that must be disposed of as they emerge from the burning zone of the process. Such gases contain noxious substances such as hydrogen sulphide or sulphur dioxide, and, as well, fine particulates. Such materials must be substantially removed before the flue gases can be vented to the atmosphere.